The present invention relates to pipes, based on polyamide (PA) and polyolefin (PO) for gas distribution, especially for combustible gases such as natural gas, air-propane mixtures and liquefied petroleum gas (LPG) and preferably natural gas, and, more precisely, pipes comprising at least one layer of polyamide and at least one layer of polyethylene. These pipes are particularly used for medium-pressure and low-pressure distribution networks, i.e. operating at pressures between 2 kPa and 1 MPa, which distribute the gas to private dwellings and to blocks of flats.
For this application, pipes made of polyamide (PA), mainly nylon-11, are known, as are pipes made of medium-density and high-density polyethylene of the MRS80 (PE80) and MRS100 (PE100) types in the sense of the ISO/DIS12162 standardxe2x80x94xe2x80x9cThermoplastics for applications as pipes and fittings: Classification and Designationxe2x80x9d. These pipes made of thermoplastic resin are used either for installing new distribution networks or for renovating existing networks consisting of metal (steel and cast iron) pipes by inserting these thermoplastic pipes into already installed metal pipes, or for replacing all or part of the thermoplastic pipes inserted into metal pipes.
Some of the mechanical properties of PA are superior to those of PE, among which may be mentioned the elastic modulus and the stress at the yield point, the instantaneous burst strength, the strain at the yield point and the offset yield stress, the resistance to creep under load and the temperature withstand performance. In contrast, the impact strength of PA is less than that of PE.
In addition to the mechanical properties indicated above and especially the creep resistance, three other important parameters should be considered, namely the long-term burst pressure, the ability to be wound on a reel or the reelability (an important parameter in storing and transporting pipes) and the chemical resistance.
Since the long-term burst pressure of a pipe correlates with the tensile properties of its constituent materials and with its creep resistance, the burst pressure of PA is superior to that of PE (for identical pipe configurations). For gas transmission and/or distribution pipes, this property is measured for a minimum lifetime of 50 years. The 50-year strength called LTHS (Long-Term Hydrostatic Strength) and the minimum 50-year strength called MRS (Minimum Required Strength) are measured according to the ISO/DTR9080 and AS 2943 and 2944 standards. For the nylon-11 marketed by the Applicant under the name RILSAN(copyright) BESN Yellow 41 TL (or hereinafter, more succinctly, Yellow nylon-11), LTHS=15 MPa; for PE 80 marketed under the name FINATHENE(copyright) black PE3802, LTHS=8 MPa; for PE 100 marketed under the name ELTEX(copyright) TU B121, LTHS=10 MPa.
Each pipe may be sized according to the burst pressure of the constituent material of the pipe, to the service pressure of the pipe (and to a safety factor). Taking 2 pipes (one made of PA and the other of PE) of the same external diameter, the SDR (Standard Dimensional Ratio=outside diameter/thickness) of the PA pipe is superior to that of the PE pipe. Thus, for a service pressure of 300 kPA (with a safety factor of 3), the SDR of the PE80 pipe is equal to 17.6 whereas that of the Yellow nylon-11 pipe is 33.
In order to assess the ability of a pipe to be wound onto a reel or its reelability, a strain is imposed on it which tends to ovalise it. This strain depends not only on the geometry of the pipe and the diameter of the reeling spindle, but also on the elastic properties of the material (in fact, if it is desired for the pipe, once unreeled, to become circular again, there should be no plastic deformation).
PA pipes are superior to PE pipes when the limiting elastic stress of each of the two materials, as well as their minimum reeling diameters, are taken into account.
The table below shows the minimum reeling diameter for pipes of various external (or outside) and internal (or inside) diameters made, on the one hand, of yellow nylon-11 with an SDR=17 and, on the other hand, of PE80 (FINATHENE(copyright) PE3802) with an SDR=11.
The pipes made of PA, being stronger than those made of PE, require less material but they reel poorly. They are particularly suited for straight portions; in contrast, more fittings are required for non-straight portions. But when the outside diameter of the pipe is imposed, for example in a case where it has to be inserted into metal pipes of an existing network, the inside diameter of a PA pipe for conveying gas will be greater than that of a PE pipe.
The chemical resistance of PA, in particular of nylon-11, to aromatic and chlorinated hydrocarbon compounds and to acids and bases is superior to that of PE. By way of example, as a comparison of the resistance of Yellow nylon-11 and PE80 immersed in a synthetic solution of natural gas condensates (a 10/20/25/25/10/10 benzene/toluene/xylene/cyclohexane/kerosene/styrene mixture, the proportions being by volume) for 72 hours gives the following results: